Let's recognize that the idea of trying to stealthily remove the buffs is a nice one. I think it would be easier to do that from Greater Invisibility, but anyway, kudoos to the OP for providing a strategy for this opponent! [I am totally serious in this compliment].
With regard to the initial rounds, I had identified, from the various posts, that the strategy was that
a) both the Sim and the original cast Dispel magic at the Wizard and his own Simulacrum until they are both spent of buffs.
b) the Drow Matron mother does something so far unspecified (but probably nothing as she isn't stealthy at all and wouldn't want to broadcast that an attack is underway...
When the dispel session have been running for long enough (probably 2 rounds to be sure no roll were missed), it's a combo of Wall of Force, a spel that can create a spherical sphere of force magic, impervious to damage except Desintegrate, that blocks moving physically through it, and Wished Dark Star, a spell that creates a sphere, up to 40 ft in radius, where no sound can exist, granting deafness and making it so no spell with a verbal component can be cast, filled with magical darkness. Any creature that enters the spell area of starts its turn inside must make a CON save or take 8d10 force damage. It lasts for 10 rounds (concentration).
The wall of force lasts for up to 10 minutes (Concentration).